1. Field of the Invention
An improved stake adapted to be driven between parallel rows of tubes within a condenser or heat exchanger bundle, in order to dampen vibration and provide support between rows of tubes. The invention resides in a tube stake that is elongated and of a soft V configuration, with a series of saddles upon each upper leg, so as to allow vertical tube expansions, without any substantial transverse motion of the tubes having lower surfaces engaging the series of saddles.
2. Brief Description of the Prior Art
It is well known that tube bundles used in heat exchanges and condensers are prone to sympathetic vibration and movement, as a consequence of temperature increases and density changes as a result of fluid velocity changes both inside and outside of the tubes. Such vibrations are of a oscillatory nature, and the oscillations can reach critical amplitudes and severely damage the tubes.
This well known problem has become more critical within condensers or heat exchangers wherein tubes originally comprised of Admiralty brass, or other relatively stiff materials, are replaced with lighter weight noble metal materials, such as titanium. A Design Guide published by the Heat Exchange Institute, 8th Edition, provides structural standards for differing tubing materials, wherein a maximum mid-span spacing between support plates perpendicular to the center line of rows of tubes is provided. By way of background, a permitted mid-span between support plates is typically on the order of between 30 inches and 50 inches, depending upon the inherent properties of the tube material, and various other design parameters such as pitch between center lines of each tube and the operating conditions of the condenser or heat exchanger. Since about 1955, with the advent of multi-spindle drills, approximately 49 holes typically are drilled in each of the tube support plates to be spaced longitudinally along the tubes, so as to define several bays. Such condensers are typically constructed so that the tube support plates space the tubes at the apices of an equilateral triangle, with the center line distance between adjacent tube being equal, in any direction. Accordingly, with respect to such condensers and heat exchangers, a free space or Lane is defined perpendicular to a line connecting the center lines of each tube which is the same, throughout the tube bundle array. Condenser tubes typically range in outer diameter between 0.75 inches and 1.25 inches, and the pitch between any two adjacent tubes is typically on the order of 0.875 inches, 1 inch, 1.125 inches, or 1.25 inches. As used hereinafter, the term Lane, is used to define the clearance space perpendicular to a line connecting the center line of two adjacent tubes. For example, for a condenser having tubes with an outer diameter of 0.875 inches, mutually spaced with a pitch dimension of 1.25 inches, and each tube having its center line at the apex of an equilateral triangle, there will be a Lane defined between adjacent horizontal tube rows of 0.208 inches, approximately. This nominal Lane dimension also is affected by the fact that the holes drilled in each tube support plate are slightly greater than the 0.875 outer dimension of each tube, with a typical tolerance for the hole in each tube plate being approximately 0.010 inches. Therefore, for a Lane of 0.208 inches, a stake may be able to separate horizontal rows of tubes up to 0.218 inches, before there is any deformation or bowing of an individual tube, at its mid-span between adjacent support plates in a given bay.
Within this environment, a representative prior art stake for a tube bundle is represented by Williams (U.S. Pat. No. 4,648,442). For further illustration of the environment for a tube stake of the present invention, Williams is incorporated herein by reference. Williams defines a support stake comprising a die-formed channel having a thickness or depth (measured from the top surface of the stake to the bottom channel surface) that essentially is identical to the Lane for a given condenser application.
The present invention is an improvement over the stake defined by Williams, in that the present invention teaches a stake with saddles defined along the upper surface of a longitudinal strip of metal bent upwardly substantially into a V shape so as to permit the lower surface of each tube in a given row to come to rest within a saddle, and thereby lock the stake against further longitudinal movement. A ligament is defined between each of the saddles defined on the upper or distal ends of the V configuration, and each saddle is spaced at the pitch of the condenser tubing. For example, where the pitch is 0.125 inches, and each tube in a horizontal row has an 0.875 inch outer diameter, the ligament will be less than 0.375 inches, with an exact dimension controlled by a desired spring-load effect. The included angle between each side of the V of the stake, and the original width of the metal strip, are used to define a vertical projection dimension that is greater than the Lane dimension. The vertical projection of the stake is measured from the bottom stake at the longitudinal bend line to the top of each distal end of the stake leg. The vertical distance between the bottom of the stake and the lowest, upper point of the upper surface of each saddle also is controllable, and is either equal to, or slightly greater than the Lane dimension. Hence, unlike the prior art stake of Williams, the present invention permits quick adjustment to the spring effect, and the amount of a vertical dimension interference fit in the Lane, simply by adjusting the included angle of the sides about longitudinal bend line; the original metal strip width; and the ligament dimension between saddles.